1. Field of the Invention
The present invention relates to an electrostatic precipitator suitable for use in a power plant, a cement plant, an industrial waste incinerator, a road or a tunnel for removing floating particles or radioactive dust, or for cleaning indoor air.
2. Description of the Related Art
The electrostatic precipitator is based upon the principle that dust is caught in a discharge portion of a machine using a high voltage. In a casing of the electrostatic precipitator, discharge electrodes (or negative electrodes) for establishing a non-uniform electric field and dust-collecting electrodes for collecting the dust are spaced apart and arranged opposite each other. A high voltage is applied between the discharge electrodes and the dust-collecting electrodes, and electricity is discharged in the air between the electrodes. Both a positive and a negative discharge may be used, a positive discharge, however, produces less ozone.
In a prior art electrostatic precipitator 100, shown in FIG. 14, a plurality of plate shaped metal dust-collecting electrodes 200 are positioned at a spacing so that their faces are parallel. A plurality of wires 400 each suspending a plurality of discharge electrodes 300 are interposed between the dust-collecting electrodes 200. The air to be treated is introduced between the electrodes, parallel with the faces of the dust-collecting electrodes 200. The discharge electrodes 300 may be supported not only by wires 400, but also by rods or pipes.
A high negative voltage is applied to the discharge electrodes 300 with respect to the dust-collecting electrodes 200 to establish a corona discharge in the air, thus forming charged zone between the dust-collecting and discharge electrodes. The air to be treated is passed through the charged zone to negatively charge dust particles m floating in the air. The charged dust particles m may then be collected and trapped by the dust-collecting electrodes 200, which are at a positive potential with respect to the dust particles m. In addition, the electrostatic precipitator 100 may be equipped with a blower for feeding the air to be treated.
In the apparatus thus constructed, as shown in FIG. 15, the dust-collecting electrodes 200 are disposed at an equal spacing a, and the discharge electrodes 300 are disposed at an equal spacing b between dust-collecting electrodes 200. When a high negative potential is applied to the discharge electrodes 300, a non-uniform electric field is established in the air between the electrodes by the corona discharge from the leading ends of the discharge electrodes 300. As a result, the ionized (charged) dust particles m are better collected by the portions of the dust-collecting electrodes 200 directly opposite the ends of the discharge electrodes 300, because of the large voltage difference between the ends of the discharge electrodes 300 and the dust-collecting electrodes 200.
In another example of the prior art, as shown in FIG. 16, an electrostatic precipitator 110 includes dust-collecting electrodes 510 having faces formed with a plurality of apertures 510a. The dust-collecting electrodes 510 are arranged at a right angle with respect to the flow direction (arrow) of the air. The electrostatic precipitator includes a plurality of discharge electrodes 610 comprised of rectangular metal plates with saw-toothed portions 610a on their edges. The discharge electrodes 610 are arranged so that their faces are in parallel with the flow direction of the air.
As shown in FIG. 15, however, the dust particles m collected by the dust-collecting electrodes 200 tend to move slightly in the air flow direction. In addition, after a layer of dust particles m have coated the collecting faces of the dust-collecting electrodes 200, the surface of the layer of collected dust particles becomes slippery and dust particles to be collected by the dust collecting electrodes easily escape the trapping action of the electric field. Moreover, once a dust particle breaks away from an area near the ends of the discharge electrodes, where the electric field is strong, it floats downstream past an area where the electric field is weak, and the electrostatic precipitator fails to provide a high cleaning percentage.
In addition, the dust particles to be collected may be partially ionized, thus becoming inversely ionized particles, as indicated by m.sup.-. The inversely ionized particles m.sup.- may be collected by the discharge electrodes 300 in the non-uniform electric field. In a case where, for example, the discharge electrodes 300 have faces opposed to the dust-collecting electrodes 200, as shown in FIG. 15, the non-uniform electric field may be established so that many inversely ionized particles m.sup.- are collected by the discharge electrodes 300 (as shown in the lower portion of FIG. 15). As a result, a clogging phenomena may occur, wherein dust particles are collected by the ionizing lines (or the discharge electrodes). This clogging phenomena causes the problem that the discharge current is weakened.
In the apparatus shown in FIG. 16, on the contrary, there is proposed a device (as disclosed in Japanese Patent Application Laid-open No. 31399/1991) for vibrating the dust-collecting electrodes 510 so as to prevent the clogging phenomenon. When this vibrator acts, however, portion of the collected dust particles m are undesirably floated in the air.
With the dust-collecting electrodes 200 arranged in parallel with the flow direction, on the other hand, the discharge electrodes 300 may be arranged in multiple stages so that the dust left uncollected by an upstream stage may be removed at a downstream stage. Unfortunately the downstream stage has a low collecting percentage. This is because larger dust particles have a higher charging capacity and are more likely to be immediately collected at the upstream side, whereas smaller dust particles have a lower charging capacity and are less likely to be collected at the upstream or downstream sides. If the spacing between the different kinds of electrodes were gradually narrowed toward the downstream side, the electric field would be accordingly intensified to collect the smaller dust particles at the downstream side. Since, however, the spacing a is unchanged at the downstream side, the efficiency is not high. Because it is structurally impossible to narrow the spacing, the efficiency is low for the large depth.
The discharge electrodes 610 shown in FIG. 16 have a one-stage structure in the flow direction, and these structures have to be arranged in multiple stages so as to enhance the efficiency. The multiple stages cause the depth of the electrostatic precipitator to be enlarged, thus raising a problem in the space required for the installation. In addition, when the plate-shaped electrodes are vertically extended, it is necessary to prevent the electrodes from bulging in a direction perpendicular to the faces of the electrodes. If recessed, on the contrary, the dust-collecting efficiency is deteriorated.